Acoustic feedback problems occur due to the fact that the output loudspeaker signal of an audio reinforcement system is partly returned to the input microphone via an acoustic coupling through the air. This problem often causes significant performance degradations in applications such as public address systems and hearing aids. In the worst case, the audio system becomes unstable and howling occurs. A state-of-the-art solution for reducing the effects of acoustic feedback is a cancellation system using adaptive filters in a system identification configuration.
Frequency shifting has been used for acoustic feedback control in audio reinforcement systems since 1950s. It can be used as a standalone system and/or it can be combined with an acoustic feedback cancellation system using adaptive filters. A spectral shifting of the loudspeaker signal in an audio system has a de-correlation effect on the reference signal from the error signal, which is useful for alleviating the generally biased adaptive filter estimation. U.S. Pat. No. 3,257,510A deals e.g. with an improved feedback control apparatus. A continuously varying phase shift affording an effective frequency shift between the input and output devices of a public address system or the like is provided, minimizing the tendency of the system to oscillate.